We use photometry from the fully cryogenic phase of the NEOWISE mission to set lower limits on the light curve amplitudes of ~21,000 Main belt asteroids (MBAs) not identified as members of collisional families. We analyze and group the data into bins with the same effective observational biases. We use the raw brightness variation distributions and the Kolmogorov-Smirnov test to draw relative conclusions on shape and rotation properties as a function of orbital elements and physical properties. After excluding low-precision photometry, our remaining sample spans a diameter range of 1 – 50 km, though 96% of the sample falls within 1– 20 km. We find no significant evidence in our sample of correlations between brightness variation and orbital properties or visual albedo. We also see that MBAs with diameters larger than ~3 – 8 km tend to have higher amplitudes (and thus more elongated shapes and/or anisotropic spin pole distributions), and we find statistically weak evidence that this trend reverses at MBA diameters larger than ~10km. The lack of a correlation between brightness variation and albedo may imply that collisional reshaping and spin pole reorientation are not sensitive to composition, at least as indicated by the surface. Also, the lack of a correlation between semimajor axis and amplitude despite collisional probabilities varying between heliocentric regions may suggest efficient dynamical mixing, efficient collisional reshaping, and/or inefficient spin pole reorientation in the 1 – 20 km diameter range.